The present invention relates to a method of locating an unknown radio-frequency transmitter by remote and passive interferometric observations.
It has been difficult to determine the locations of unknown transmitters which serve as sources of interference to geosynchronous satellite communications systems because of the wide range of potential locations of the interfering transmitters.
One technique which has been used is a time difference of arrival calculation (TDOA) by which the propagation time for the signal uplinked from the unknown transmitter through a particular satellite to a receiving station is compared with that through a neighboring satellite. The signal from the unknown transmitter propagates along different paths to the two satellites and arrives after a delay which is different for the two paths because of their unequal length. Given that the position of the two satellites relative to a receiving station is known, the difference in time of arrival over the two different paths will serve to localize the unknown transmitter location to a curve on the surface of the Earth. This curve is called a delay line of position (LOP) which is the intersection of a hyperbola of revolution with the surface of the Earth. For a typical case of two neighboring geosynchronous satellites, the lines of position of constant time difference value (that is, isodelays) are slightly curved lines running roughly North-South. The exact value of the observed delay depends on the precise geometry including the locations of the two satellites, atmospheric propagation, and instrumental delays in the satellite transponders and the receiving station equipment. Realization of a TDOA measurement requires a two-antenna receiving station capable of accurately estimating the differential delay between the two paths. A LOP determined by TDOA measurements is useful but not sufficient to accurately pinpoint the location of the unknown transmitter since the TDOA measurement localizes the position of the unknown transmitter to only a LOP on the surface of the Earth.
Some additional technique is needed to locate more accurately the unknown transmitter in order to locate the source of interfering signals which disrupt satellite communications systems.
Additionally, a technique is needed to enhance the accuracy of the initial measurements.